This invention relates to lens systems, and in particular it relates to lens systems for use with infra-red radiation.
As is well known, all optically transmissive materials have a refractive index, n(.lambda.) and a corresponding reciprocal dispersive power or V-value, which are associated respectively with the wavelength .lambda. and a defined wavelength interval .DELTA..lambda.. The variation of the refractive index with wavelength results in a single lens exhibiting chromatic abberation i.e. the degree of refraction being dependent upon the wavelength of the refracted radiation. In general, if it is required to bring two different wavelengths to a common axial focus, then two optical materials of different V-values are required. Similarly three materials of different V-values are generally required to focus three different wavelengths and so on.
Optical systems for use with the far infra-red band (generally 8 to 12 .mu.m) usually employ germanium as the sole optical material. Although only one wavelength may be brought to a common focus within a system employing only germanium elements, it is generally recognised that due to the relatively high V-value of germanium, about 1000, chromatic dispersion will be small enough to ignore in most applications. However, several more recent applications and the use of more complex designs require the chromatic dispersion to be corrected so that for example two wavelengths may be brought to a common focus. In these cases another material such as zinc selenide (ZnSe) or zinc sulfide (ZnS) is combined with the germanium. Systems such as this are generally used when two wavelengths within the far infra-red band, such as eight .mu.m and eleven .mu.m, are to be brought to a common focus.
Recent advances in electro-optical systems have led to a requirement that the optical system be adapted to bring radiation from both 3-5 .mu.m and 8-12 [m wavebands to a common focus within the limit prescribed by the "Rayleigh Quarter-Wave Rule". Current methods of achieving this involve the use of at least three different materials in the optical system, generally comprising ZnS, ZnSe and Ge. It has not previously been though possible that a lens system satisfying these requirements can be produced from only two materials.